Programmable logic controller module assembly and locking system

ABSTRACT

A system of interlocking modules for use with a programmable logic controller. The system utilizes a plurality of modules, e.g. input/output modules, that are mechanically interlinked to obviate the need for a rack or back plane. Each module includes a movable plug portion that selectively may be moved into engagement with a corresponding plug receptacle of the next adjacent module. The mechanical interlocking features and the plug portions are designed to permit insertion and removal of individual modules disposed between adjacent modules, without moving either of the adjacent modules.

FIELD OF THE INVENTION

The present invention relates generally to controller assemblies, such as those used in a programmable logic controller (PLC) system, and particularly to a modular component for a PLC system that facilitates attachment and detachment of individual modules, such as I/O modules, processor modules, communication modules and power supplies.

BACKGROUND OF THE INVENTION

Programmable logic controllers are used throughout industry to control and monitor a wide range of machines and other movable components and systems. Conventional PLC systems utilize a plurality of modules, e.g. input/output modules, that are mechanically mounted in a rack system and electronically connected along a back plane. The individual modules can be interchanged or replaced by disconnecting them from the rack and/or back plane. It would be advantageous to eliminate the mounting components, e.g. rack and back plane, and to provide modules that were in an interlocking, modular form.

At least one attempt has been made to construct modules that may be interconnected to each other without the use of a rack or back plane. In this particular embodiment, each module includes a stationary plug and a stationary plug receptacle designed for mating engagement with the stationary plug of a next adjacent module. Additionally, each module includes a housing having a plurality of tongs on one side and a plurality of openings on the opposite side to receive the extended tongs of the next adjacent module. Each module also includes a mounting mechanism for mounting along a DIN rail.

Thus, an individual module can be attached to an adjacent module by moving the individual module along the DIN rail until the tongs and plug connector engage the openings and plug connector of the next adjacent module. Once engaged, locking mechanisms are used to hold the module prongs within the corresponding openings of the next adjacent module.

This configuration does not permit mechanical interlocking of adjacent modules independent of forming an electrical connection between modules. Additionally, the design does not permit the removal of an individual module that is sandwiched between adjacent modules without upsetting the mounting of additional I/O modules. The mechanical interlocking system requires that a module be moved laterally sufficiently far to clear its engagement prongs from the openings of the next adjacent module. To permit this lateral movement, all of the modules positioned on one side of the subject module must be moved to provide sufficient clearance for removal and/or insertion of the subject module. Once the subject module is engaged, the string of modules moved out of the way must be reengaged.

It would be advantageous to have a compact, rackless, modular PLC system utilizing modules that could be selectively, mechanically interlocked and yet readily inserted or removed regardless of whether the module was disposed between adjacent modules.

SUMMARY OF THE INVENTION

The present invention features a system for interlocking modules, e.g. input/output modules, of a programmable logic controller. The system includes a plurality of modules for use in a PLC system in which each module includes a first interconnection feature and a second interconnection feature. The first interconnection feature is configured for selective engagement with the second interconnection feature of an adjacent module. Each module also includes a connector plug portion on one side of the module and a corresponding plug portion on the other side of the module. The connector plug portion is designed to provide an electrical interconnection with the adjacent module. Similarly, the corresponding plug portion is configured to receive a connector plug portion from another adjacent module positioned on the other side of the subject module. At least one of the connector plug portion and the corresponding plug portion is mounted for reciprocal movement into and out of electrical connection while the adjacent modules are mechanically interconnected.

According to another aspect of the invention, a PLC module is configured for mounting along a common mounting structure. The PLC module includes an outer housing having a back portion, a front portion, a left side and a right side. The PLC module further includes an attachment mechanism designed to secure the outer housing to the common mounting structure, such that the back portion lies adjacent the mounting structure. Additionally, the module includes a plurality of interconnection features arranged to permit sliding engagement with a pair of adjacent PLC modules that also may be connected to the mounting structure. The interconnection features are disposed adjacent both the left side of the housing and the right side of the housing. Furthermore, the plurality of interconnection features are oriented to permit removal of the PLC module while the pair of adjacent PLC modules remain in position along the mounting structure.

According to a further aspect of the invention, a method is provided for interlocking a plurality of modules in a PLC system. The method includes mechanically locking a first module with a second module. Additionally, the method includes independent formation of an electrical interconnection between the first module and the second module.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and:

FIG. 1 is a perspective view of a system of PLC system modules, according to a preferred embodiment of the present invention;

FIG. 2 is a front view of the modules illustrated in FIG. 1;

FIG. 3 is a rear view of the modules illustrated in FIG. 1;

FIG. 4 is a perspective view of an individual module of the type illustrated in FIG. 1, taken generally from the right side.

FIG. 5 is a right side view of the module of FIG. 4;

FIG. 6 is a perspective view of the module of FIG. 4, taken generally from the left side;

FIG. 7 is a left side view of the module of FIG. 4;

FIG. 8 is a top view of the module of FIG. 4;

FIG. 9 is a perspective view of an interlocking lever of the module, according to a preferred embodiment of the present invention;

FIG. 10 is another perspective view of the lever of FIG. 9; and

FIG. 11 is a perspective view similar to that of FIG. 10, but showing the plug connector adjacent the lever.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring generally to FIGS. 1 and 2, a system 20 of interlocking modules 22 is illustrated, according to a preferred embodiment of the present invention. Interlocking modules 22 are, by way of example, input/output modules, processor modules, communication modules and power supplies, of the type used with a PLC system. Modules 22 may be mounted along a mounting structure 24, such as a DIN rail 26 or planar surface.

In the embodiment illustrated, three modules 22 have been illustrated as mounted along DIN rail 26. The selection of three modules is for the purpose of enhancing explanation. The illustration and discussion of three interlocking modules 22 should not, in any way, be limiting. For example, fewer modules or many additional modules can be incorporated into system 20 without departing from the scope of the invention. Additionally, the modules are typically connected to a controller, such as a programmable logic controller.

Each module 22 includes an outer module case or housing 28 having a back portion 30 (see FIG. 1), a front portion 32, a left side 34 and a right side 36. Additionally, each module 22 includes a top 38 and a bottom 40. The reference to left, right, top, bottom, back and front are based on the orientation of system 20 in FIG. 1. However, those terms are used to facilitate explanation and should not be construed as limiting the orientation of interlocking modules 22.

In the preferred embodiment, each module 22 includes interlocking features for selective connection to adjacent modules without the use of a rack or back plane. Specifically, each module includes a first interconnection feature 42 and a second interconnection feature 44. In the most preferred embodiment, first interconnection feature 42 comprises an extension 46 having a groove 48. Second interconnection feature 44, on the other hand, comprises a protrusion or tongue 50 designed to matingly engage groove 48. As illustrated, an extension 46 and corresponding groove 48 may be disposed along the left side of housing 28 proximate top 38 and bottom 40. A tongue 50, on the other hand, may be disposed along the right side of housing 28 proximate top 38 and bottom 40. The alternating tongues and grooves of adjacent modules 22 permit the interconnection of multiple modules.

For example, the illustrated modules 22 can be labeled A, B and C, with module A being left most, module B being central, and module C being right most. In this particular example, central module B is connected with the next adjacent module to its left, A, when the tongue 50 of module A is slidingly engaged with the groove 48 of module B. Similarly, the next adjacent module to the right of module B, labeled as module C, can be joined by sliding groove 48 of module C along tongue 50 of module B. These mechanically interlinked modules 22 can be mounted along mounting structure 24, e.g. DIN rail 26.

Preferably, the interconnection features, such as grooves 48 and tongues 50 are oriented in a direction generally transverse to the plane in which mounting structure 24 lies. Thus, a centrally located module, such as module B can be released from mounting structure 24 and removed from the adjacent modules A and C by sliding the module away from mounting structure 24 until its interconnection features 42 and 44 clear the adjacent modules. This eliminates the need to remove additional modules when removing or replacing a centrally located module.

Additional features of each module can now be described. As illustrated in FIG. 2, front portion 32 includes indicator 52 that may comprise a plurality of LEDs 54. Additionally, a removable cover 56 is disposed over a plurality of input/output terminals 58 as illustrated in the partially broken away section of FIG. 2.

Back portion 30 is illustrated best in FIG. 3. Back portion 30 includes a DIN rail mounting area 60 for receiving a DIN rail. Additionally, an upper DIN latch 62 and a lower DIN latch 64 are slidably mounted in slides 66. Upper DIN latch 62 and lower DIN latch 64 each have a biasing mechanism 68 that biases the latch to an open or a closed position. Specifically, each biasing mechanism 68 includes a pair of resilient prongs 70 that slide past a stationary abutment 72 as the latch moves along it corresponding slide 66. The resilient prongs 70 are shaped to require a certain amount of force to bias the latch from an open to a closed position and vice versa.

Additionally, each module 22 preferably includes an upper mounting foot 74 and a lower mounting foot 76. Mounting feet 74 and 76 may be used to mount each module 22 to a panel, typically a planar panel used in lieu of DIN rail 26.

As illustrated in FIGS. 4 and 5, the right side of each module 22 preferably includes a side wall 78. Side wall 78 includes an opening 80 through which a connector plug portion 82 is exposed. Connector plug portion 82 is designed for mating engagement with a corresponding plug portion of the next adjacent module 22 to form an electrical interconnection between adjacent modules, as described more fully below. Connection plug portion 82 may, for instance, be in the form of a plug receptacle having a plurality of conductors 84. Also disposed within opening 80 is a ground contact 86 that meets with corresponding ground contacts in adjacent modules. A locator 87, such as rounded male protrusions, can be disposed along side wall 78, as illustrated. Locators 87 help position adjacent modules 22 to facilitate electrical and mechanical interconnection.

Furthermore, a locking notch 88 is formed in tongue 50 to aid in the mechanical interconnection of adjacent modules once those modules are electrically interconnected. A lever lock recess 90 also may be formed in side wall 78 to further interlock adjacent modules once mechanical and electrical interconnection has been formed.

As illustrated in FIGS. 6 and 7, the left side 34 of each module 22 typically exposes a generally open interior 92 in which a printed circuit board 94 is mounted, for instance, by a latch mechanism 96. Additionally, a mating connector plug portion 98 is electrically coupled with circuit board 94 by a flexible connector 100, such as a flexible ribbon cable.

Mating connector plug portion 98 may be a male plug connector designed for mating engagement with plug portion 82. Mating plug portion 98 may include, for instance, a plurality of receptacles 102 arranged to receive conductors 84 of plug portion 82 to form electrical interconnection for proper transfer of signals from one module to another. To ensure proper mating of plug portions 82 and 98, a plurality of mating ridges 104 (see FIG. 4) and grooves 106 (see FIG. 6) may be formed in plug portions 82 and 98, respectively. Preferably, a pair of locator recesses 107 are positioned to receive male locators 87 and to facilitate alignment of adjacent modules as well as plug portions 82 and 98.

At least one of connector plug portion 82 and mating connector plug portion 98 is movable to permit selective engagement with the corresponding plug portion of the next adjacent module 22. Preferably, the movable plug portion is movable in a direction generally transverse to the direction in which adjacent modules 22 are slidingly engaged via mating grooves 48 and tongues 50. In other words, the movable plug portion preferably moves in a direction generally parallel to mounting structure 24.

In the illustrated embodiment, the male plug portion 98 is mounted for movement, and connector plug portion 82 is mounted in a stationary position. Thus, male plug portion 98 is connected to printed circuit board 94 via ribbon cable 100, and female plug portion 82 is affixed to stationary PC board 94, generally on an opposite side thereof. When adjacent modules 22 are mechanically interlinked, the male plug portion 98 of one module may be moved into mating engagement with the female plug portion 82 of the module to its left. Similarly, the female plug portion 82 may be engaged by the male plug portion 98 of the next adjacent module 22 disposed to its right. Thus, an electrical interconnection and mechanical interconnection can be formed independently between adjacent modules, e.g. input/output modules. Also, the electrical interconnection can be disengaged independently of the mechanical interconnection. Furthermore, upon mechanical release and electrical release (see description below), an individual module 22 may be released from mounting structure 24 and removed without moving the modules adjacent its left and right.

A preferred mechanism for moving plug portion 98 into and out of engagement with a corresponding plug portion 82 is a pivotable lever 108. Pivotable lever 108 is mounted to a pivot pin 110 extending into open interior 92 from back portion 30. Pivotable lever 108 includes a handle portion 112 that extends through top 38, as best illustrated in FIG. 8.

Both top 38 and bottom 40 include a plurality of ventilation slots 114, but top 38 also includes a lever slot 116. Lever slot 116 includes a narrowed region 118 that engages handle 112 when plug portion 98 is moved into engagement with plug portion 82 of an adjacent module 22. Additionally, an open stop 120 and a closed stop 122 extend into lever slot 116. Open stop 120 maintains lever 108 in an open or disengaged position, and closed stop 122 maintains lever 108 in a closed or engaged position when plug portion 98 is moved into engagement with plug portion 82. The closed stop 122 aids in maintaining the lever 108 in an engaged or closed position even in industrial environments with substantial vibration.

As best illustrated in FIG. 7, plug portion 98 is mounted to pivotable lever 108 between pivot pin 110 and handle portion 112. Preferably, a plug guide 124 extends from top 38 into open interior 92. Plug portion 98 includes a corresponding plug rail 126 that is held within plug guide 124 for reciprocal motion along a single axis of movement. Furthermore, plug portion 98 is pivotably and slidably mounted to lever 108 to permit linear movement of plug portion 98 along plug guide 124 during engagement and disengagement. This type of mounting ensures that plug portion 98 moves in a linear fashion into engagement and out of engagement with a corresponding plug portion 82.

The specific details of pivotable lever 108 and its connection to plug portion 98 may be explained more fully with reference to FIGS. 9-11. As illustrated, lever 108 includes an aperture 128 designed for pivotable engagement with pivot pin 110. Additionally, lever 108 includes a nail head slot 130 designed to receive a plug nail head 132 extending from plug portion 98. Plug nail head 132 includes a neck 134 and an expanded portion 136. When plug portion 98 is connected to lever 108, neck 134 permits both pivotable and sliding motion of plug portion 98 within nail head slot 130. However, the nail head slot 130 is captured between expanded portion 136 and the main body of plug portion 98 to prevent inadvertent detachment. Additionally, the mounting of plug rail 126 in plug guide 124 ensures that plug portion 98 does not slide beyond the reach of nail head slot 130.

Lever 108 also includes a lever case lock protrusion 138 designed for engagement with lever lock recess 90 (see FIGS. 4 and 5) to help prevent movement of adjacent modules 22 once their plug portions 98 and 82 are engaged. Lever 108 also includes a locking ledge 140 disposed for engagement with locking notch 88 of the next adjacent module 22 when adjacent modules are mechanically and electrically interconnected.

Handle portion 112 may be connected to a central body portion 142 by a lever spring 144. Lever spring 144 provides added flexibility in handle portion 112 as it moves around open stop 120 and closed stop 122 to engage narrowed region 118. The lever spring 144 further aids in maintaining lever 108 in a closed position by preventing inadvertent movement past closed stop 122 towards an open position.

In operation, adjacent input/output modules 22 may be mounted along a mounting structure, such as DIN rail 26. The adjacent modules 22 are mechanically interlocked by slidably engaging corresponding tongues 50 with grooves 48. The grooves 48 and tongues 50 are arranged to permit adjacent modules 22 to be slid into engagement in a direction generally transverse to the DIN rail 26. This permits individual modules to be removed when sandwiched between adjacent modules, without removing any adjacent modules.

Furthermore, system 20 permits adjacent modules 22 to be electrically interconnected independent of any mechanical interconnection. Once adjacent modules are mechanically interconnected, the electrical interconnection may be formed simply by pivoting lever 108 and moving male plug portion 98 into engagement with female plug portion 82 of the next adjacent module 22.

Similarly, an individual module 22 may be removed simply by disengaging its male plug portion 98 from the adjacent female plug portion 82. If there are any modules 22 mounted adjacent the right side 36, the lever 108 of that module is moved to release its plug portion 98 from plug portion 82 of the subject module 22. The subject module may then be released from DIN rail 26 by separating its latches 62 and 64. At this point, the subject module 22 is slid away from the DIN rail 26 and mechanically disconnected without moving either of the adjacent modules 22. This ability to quickly and easily remove and replace individual modules without dismantling the system, substantially reduces downtime when servicing and repairing PLC systems utilized in an industrial environment.

Furthermore, it is preferred that the electrical connection system be designed as a floating connector system to minimize detrimental effects of the relatively harsh conditions, such as vibration, that can occur in an industrial environment. This is accomplished by permitting movement of plug portion 98 with respect to its mechanical mounting points, and by connecting plug portion 98 to stationary PC board 94 via flexible ribbon cable 100.

Specifically, plug nail head 132 is allowed to freely pivot and slide along nail head slot 130. Additionally, a relief gap 146 is disposed between plug guide 124 and plug rail 126 of plug portion 98, as illustrated best in FIG. 7. The combination of flexible ribbon cable 100, nail head slot 130 and relief gap 146 permit plug portion 98 to be relatively free-floating, even after engagement with female plug portion 82.

Upon engagement, a given plug portion 98 effectively becomes part of the next adjacent module 22 that carries the female plug portion 82 to which it is engaged. The given plug portion 98 is substantially isolated from shock and vibration incurred by the module 22 to which it is connected via flexible ribbon cable 100. This arrangement of interconnection features permits relative motion between adjacent modules 22 while minimizing fatigue or other damage to the connected components.

To accommodate this preferred, free-floating design, a guidance system 150 is utilized to ensure smooth engagement of plug portion 98 and the corresponding female plug portion 82 when lever 108 is moved to its closed or engaged position. Guidance system 150 may include a variety of features to help move plug portion 98 into full engagement with plug portion 82. For example, preferred features include a plurality of tapered corner pins 152, as illustrated best in FIG. 11, that help guide and align plug 98 as it begins to enter female plug portion 82. In addition, plug portion 98 includes a chamfered leading edge 154 that joins and cooperates with tapered corner pins 152 in guiding plug portion 98 into engagement with female plug portion 82. Female plug portion 82 similarly includes a chamfered receiving edge 155. Guide grooves 106 also include expanded or chamfered lead openings 156 that cooperate with inwardly chamfered ends 158 on mating ridges 104 of plug portion 82 (see FIGS. 4 and 5). All of these guidance system features cooperate to ensure smooth and consistent engagement and disengagement of plug portion 98 with female plug portion 82 while accommodating the free-floating design of the electrical connector system.

It will be understood that the foregoing description is of a preferred exemplary embodiment of this invention, and that the invention is not limited to the specific form shown. For example, the module housings may be made in a variety of configurations and from a variety of materials; the mechanical interlocking features may be integrally molded with the housing or separately attached; a variety of electrical connections can be utilized for connecting adjacent modules; flexible connectors other than ribbon cable can be utilized; and one or both of the plug connectors can be movable to facilitate electrical connection. These and other modifications may be made in the design and arrangement of the elements without departing from the scope of the invention as expressed in the appended claims. 

What is claimed is:
 1. A system for interlocking modules of a programmable logic controller, comprising a plurality of modules for use in a PLC system in which each module includes: a first interconnection feature and a second interconnection feature, the first interconnection feature of each module being configured for interconnection with the second interconnection feature of a first adjacent module wherein the first interconnection feature comprises an elongate groove and the second interconnection feature comprises an elongate tongue; a connector plug portion to provide an electrical interconnection with the first adjacent module; and a corresponding plug portion configured to receive a connector plug portion of a second adjacent module, wherein at least one of the connector plug portion and corresponding plug portion is mounted for reciprocal movement into and out of the electrical connection, while the first and second adjacent modules are interconnected therewith.
 2. A system for interlocking modules of a programmable logic controllers, comprising a plurality of modules for use in a PLC system in which each module includes: a first interconnection feature and a second interconnection feature, the first interconnection feature of each module being configured for interconnection with the second interconnection feature of a first adjacent module; a connector plug portion to provide an electrical interconnection with the first adjacent module; and a corresponding plug portion configured to receive a connector plus portion of a second adjacent module, wherein at least one of the connector plug portion and corresponding plug portion is mounted for reciprocal movement into and out of the electrical connection, while the first and second adjacent modules are interconnected therewith, wherein the connector plug portion is mounted on a pivotable lever.
 3. The system as recited in claim 2, wherein the connector plug portion is substantially free-floating.
 4. The system as recited in claim 2, wherein each module includes a printed circuit board.
 5. The system as recited in claim 4, wherein in each module the connector plug portion is connected to the printed circuit board by a flexible connector.
 6. The system as recited in claim 2, wherein each first interconnection feature comprises a groove and the second interconnection feature comprises a tongue sized for sliding engagement with the groove.
 7. The system as recited in claim 6, wherein the connector plug portion is movably mounted within the module for reciprocal movement in a direction generally transverse to the direction of sliding engagement.
 8. The system as recited in claim 7, wherein the connector plug portion is mounted on a pivotable lever.
 9. The system as recited in claim 8, wherein the connector plug portion is pivotably mounted to the pivotable lever.
 10. The system as recited in claim 9, wherein each module includes a plug guide slidably coupled to the connector plug portion to limit twisting of the connector plug as the pivotable lever is moved.
 11. The system as recited in claim 9, wherein each module includes a stop positioned to maintain the pivotable lever in a closed position.
 12. The system as recited in claim 9, wherein each module includes a stop positioned to maintain the pivotable lever in an open position.
 13. A PLC module configured for mounting along a common mounting structure, comprising: an outer housing having a back portion, a front portion, a left side and a right side; an attachment mechanism configured to mount the outer housing to the common mounting structure, such that the back portion lies adjacent the common mounting structure; a plurality of interconnection features arranged to permit sliding engagement with a pair of adjacent PLC modules connected to the common mounting structure and disposed adjacent the left side and the right side, wherein the plurality of interconnection features are oriented to permit selective insertion and removal of the PLC module while the pair of adjacent PLC modules remain in position along the common mounting structure; and an electrical connector that is selectively engageable with at least one of the pair of adjacent PLC modules subsequent to the sliding engagement with the at least one of the pair of adjacent PLC modules.
 14. The PLC module as recited in claim 13, wherein the plurality of interconnection features are integrally molded with the outer housing.
 15. The PLC module as recited in claim 13, further comprising a plug connector and a plug receptacle to permit communication of signals to adjacent PLC modules.
 16. The PLC module as recited in claim 15, wherein the plug connector is movable with respect to the housing.
 17. A PLC module configured for mounting along a common mounting structure, comprising: an outer housing having a back portion, a front portion, a left side and a right side; an attachment mechanism configured to mount the outer housing to the common mounting structure, such that the back portion lies adjacent the common mounting structure; a plurality of interconnection features arranged to permit sliding engagement with a pair of adjacent PLC modules connected to the common mounting structure and disposed adjacent the left side and the right side, wherein the plurality of interconnection features are oriented to permit removal of the PLC module while the pair of adjacent PLC modules remain in position along the common mounting structure; a plug connector and a plug receptacle to permit communication of signals to adjacent PLC modules, wherein the plug connector is movable with respect to the housing; and a printed circuit board mounted to the outer housing, wherein the plug connector is connected to the printed circuit board by a ribbon cable.
 18. A PLC module configured for mounting along a common mounting structure, comprising: an outer housing having a back portion, a front portion, a left side and a right side; an attachment mechanism configured to mount the outer housing to the common mounting structure, such that the back portion lies adjacent the common mounting structure; a plurality of interconnection features arranged to permit sliding engagement with a pair of adjacent PLC modules connected to the common mounting structure and disposed adjacent the left side and the right side, wherein the plurality of interconnection features are oriented to permit removal of the PLC module while the pair of adjacent PLC modules remain in position along the common mounting structure; and a plug connector and a plug receptacle to permit communication of signals to adjacent PLC modules, wherein the plug connector is movable with respect to the housing, further wherein the plug connector is mounted to a pivotable lever.
 19. A method for interlocking a plurality of modules in a PLC system, comprising: mechanically interlocking a first module with a second module; independently forming an electrical interconnection between the first module and the second module; mechanically interlocking a third module with the second module on an opposite side of the second module from the first module; mounting the first, second and third modules along a common mounting structure; independently forming an electrical interconnection between the second module and the third module; releasing the second module from the first and third modules; and removing the second module without moving the first and third modules with respect to the common mounting structure.
 20. The method as recited in claim 19, wherein independently forming includes moving a lever to force a movable plug of the second module into a plug receptacle of the first module.
 21. The method as recited in claim 20, wherein independently forming includes moving the movable plug in a direction generally parallel with the common mounting structure.
 22. The method as recited in claim 20, wherein mechanically interlocking includes sliding first and second modules into engagement via a tongue and a groove.
 23. The method as recited in claim 19, further comprising mounting a movable plug connector to a pivot arm on the second module and mounting a corresponding plug connector to the first module, wherein independently forming includes moving the movable plug connector into engagement with the corresponding plug connector.
 24. The method as recited in claim 19, wherein independently forming includes forming a free-floating electrical connection to the first module. 